Traditionally lithium rechargeable cells are first charged at constant current until a critical voltage is reached (the termination voltage) and then are charged at constant voltage until the charging current tapers to some minimal value, at which point charging is terminated. Typical current and voltage profiles of this charging method are shown in FIG. 1, and are known in the art. For example, with a 1000 mA*hr cell the initial charge current would be typically 1000 mA (known as a "1 Capacity" or "1C") rate and the charge method would transition to constant voltage when 4.2 volts was reached at the cell terminals. Charging would be terminated when the current tapered down to 25 mA typically. This is the type charging method recommended by most lithium ion cell manufacturers, and is widely used for recharging lithium cells.
A disadvantage of this type of charging system is that during the constant current charging portion of the charge cycle, the current should be set at a 1C rate. A 1C rate is used because it optimizes battery performance by maximizing the number of times that the battery can be recharged without losing capacity. This 1C requirement presents a problem, however, in that any one particular charger is designed to optimally charge only one particular battery. A standard cellular phone, for example, may have both a normal battery and an extended life battery. The standard battery might be a 500 mA*hr hour battery, while the extended life battery is a 1000 mA*hr battery. If the engineers design a charger to optimally charge the normal battery, the charger will source 500 mA. If a user inserts the extended life battery, a 500 mA charge current will not optimally charge the battery. Thus, to optimally charge both batteries, the user would need two different chargers! This is both expensive and cumbersome.
One prior art method used to avoid this problem is a charger design that is capable of "adjusting charge rates". In other words, charger is designed to change the charge current level depending upon the battery being charged. To accomplish this, the charger must first be able to identify what kind of cell is being charged. One way to do this is by including a "code resistor" in the battery. The charger reads the value of the code resistor and adjusts the current accordingly. For example, a 10 k resistor might correspond to a 1000 mA charge current. The problem with this method of identification is that it requires both an additional component--the resistor--and at least one additional battery terminal.
Another method of battery identification is the use of a programmable memory device. If a memory device is built into the battery, that device can store identifying information like capacity, charge current, number of charge cycles, etc. Again, this solution is very expensive and requires at least one additional battery terminal.
There is thus a need for an improved universal battery charger.